There have been very many different prior art proposals for methods and apparatus for the generation of smoothness and gloss at the surface of paper webs. One typical modern proposal is to be found in European patent application publication No. 245 250. This prior art application describes a process for producing gloss and smoothness on a surface of a paper web by providing a finishing apparatus comprising a smooth metal finishing drum and a resilient backing roll pressed against the drum at a force up to 700 KN/M (4000 pounds/linear inch) to form a nip with pressure against the paper of at least 13,780 KN/M.sup.2 (2000 pounds/square inch). The web of papermaking fibers having a moisture content of from 3% to 7% of the bone dry weight of the fibers is advanced through the nip between the smooth metal finishing drum and the resilient backing roll at a speed which results in the web dwelling in the nip from 0.3 millisecond to 12 milliseconds. During this the drum is heated to a surface temperature having a value no less than 20.degree. below a value determined by a complicated exponential formula involving the initial temperature of the web just prior to entering the nip, the dwell time of the web in the nip and the moisture content of the fibers in the web in weight percent of the bone dry fiber weight.
Typical of this process and of other prior art processes is the heating of the web in the nip between two rollers with simultaneous compression of the web in this nip.
The aforementioned European paten application also recognizes that cellulosic fibers, such as papermaking fibers, appear to exhibit thermoplastic properties and in particular appear to have a glass transition temperature "Tg" above which the fibers become much more flexible and moldable when subjected to pressing forces. It is noted in this reference that the Tg of cellulose in paper is greatly dependent upon the moisture content of the paper. The disclosure of this reference is incorporated herein by reference as far as it relates to the properties of papermaking materials with respect to their glass transition temperature and the effect of their moisture content thereon.
Another reference which teaches a method of generating smoothness and gloss on a paper web is described in European patent application with the publication number 0 341 457.
The calender arrangement described here includes two roll pairs defining respective nips through which the paper web runs in series. Each roll pair is used to treat a respective surface of the web and comprises a high temperature hard roll and a soft roll. As seen in the direction of movement of the paper web a respective cooling device precedes each roll pair and serves the purpose of cooling the web prior to it being heated in the roll gaps. The aim is to restrict the heating of the interior of the paper web in the roll gaps and thus the associated partial plastification of the interior of the paper web. In this way the heating is restricted to the surface of the web so that the surface fibers remain compressed after the web leaves the roll gap. It can be seen from the foregoing description that this reference also teaches a method of generating smoothness and gloss where the surfaces of the web are smoothed using heated rolls with the simultaneous application of pressure.
A further prior art reference in this field is German Offenlegungsschrift DE-OS 38 15 446. This reference is directed to the generation of smoothness and gloss by a rubbing effect on the surface of the paper web. More specifically the reference discloses an apparatus for generating smoothness and gloss on webs of paper, textile and the like and includes a polished metal smoothing surface along which the web can be moved under the action of pressure and friction. The movement of the web along the surface can be effected by means of a roll with a resilient coating. Through the polished smoothing surface the surface of the web is effectively ironed smooth. It is explained in the reference that the effect which generates smoothness and gloss relates to a plastification of the fiber layers of the web close to the surface. In order to enhance the effect that is sought, it is said to be of advantage for the web to be moistened and/or heated at the side to be smoothed. In particular it is recommended that the smoothing surface and/or the roll should be subjected to temperature control during the smoothing process, i.e. heated or cooled, with the temperature control taking place in dependence on the heat which is in any event generated by friction at the smoothing surface and which can be considerable. That is to say the cooling of the polished metal smoothing surface which is effected from the rear is intended not to cool the surface of the web but rather to prevent the smoothing surface from being overheated as a result of the friction that is generated. DE-OS 38 15 446 describes various types of heating arrangement, for example a radiative heating device which heats the rear side of a sheet metal smoothing surface along which the web is drawn, and a preheated roll which heats the surface of the web prior to it entering into the gap between the smoothing surface and the roll with the resilient coating. In some embodiments a cooling device is provided for cooling the paper web after it has been smoothed. As will be seen from the foregoing this reference again relates to a method in which the smoothness is generated by the simultaneous application of heat and pressure to one or both surfaces of the paper web.
It is disadvantage of the above described so-called temperature gradient methods that the roll temperature or the press forces must be increased to ensure adequate smoothness or gloss because of the increasingly shorter swell time in the roll gap which is brought about by the high speeds of production plants of over 1000 m/min. However, an increase of the press forces leads to a reduction of the web thickness which is often undesired. Furthermore, even when smoothing is carried out with heated rolls, it can be observed that the fibers at least partially re-erect themselves again after leaving the roll gap and this is associated with a loss of smoothness and gloss.
One proposal for overcoming this latter difficulty is described in International Patent Application PCT/EP89/00701 published under the International Publication Number WO 90/07027. Here the smoothing of a paper web is effected in the nip of a roll pair under the action of pressure, moisture and heat. The method described is characterized in that the surface of the paper or cardboard web to be smoothed is treated during smoothing in the press, gap or nip at a temperature above the glass transition temperature of the material, and in that the paper or card web is then cooled after leaving the press gap or nip to a temperature and moisture content beneath the glass transition point of the material within a time of 20 to 60 milliseconds of leaving the press gap. The specification thus aims at reducing the increase in roughness of the paper web after it has been smoothed in the press gap or nip between the heated rolls. The cooling of the surface of the web can be effected by contact with a chilled surface or by direct contact with a cooling gas.
A process of this latter kind admittedly represents a substantial improvement over the prior art proposals but is nevertheless still associated with certain disadvantages. In order to effect the cooling within a short time interval at a high speed of the web the cooling device must be moved fairly close to the heated rolls. To prevent heat transfer from the rolls to the cooling device it can be necessary to insert partition walls or shields between the rolls and the cooling device. This can result in a congested arrangement which can hinder servicing and repair work. Moreover the problem still exists that it is necessary to provide heated rolls, and heated rolls are in their own right somewhat problematic since hot fluid or gas is invariably required in the interior of the rolls to keep them at the correct surface temperature, thus giving rise to considerable safety problems. Outside heating of rolls is also known but requires space for the heating apparatus and enlarges the space between the heating rolls and neighboring apparatus. In addition heated rolls can suffer local distortion and temperature differences as a result of their design and these can in turn lead to marking of the surface of the paper web.
The principal object underlying the present invention is to provide a method and a corresponding apparatus for the generation of smoothness and gloss on paper which essentially precludes a re-erection of fibers after the smoothing process and which also provides freedom in the design of the heating apparatus that is required, so that, if the designer wishes, heated rolls can be avoided in the realization of the invention.